1. Field of the Invention
The present invention is related to an analytic process using porous silicon for producing a variable refractive index, as well as to an analytic device for such a process according to the introductory part of claim 8.
2. The Prior Art
Porous silicon (PS) is a promising material for applications in sensorics (gas sensors, humidity sensors, bio-sensors) because of its compatibility with highly developed Si-microelectronics as well as simple manufacturability at favorable cost, whereby use is made of the large inner surface of the material (up to a few 100 m.sup.2 /cm.sup.3) and the microstructure. Furthermore, layered systems made from PS are excellently suitable for producing optical filters and mirrors as well as wave conductors at favorable cost, whereby air is present in the pores of the PS and the refractive index of the PS is fixed in the course of production by substrate doping, the density of the etching current and the composition of the etching solution.
Porous silicon (PS) consists of a sponge-like structure of silicon crystallites, which is traversed by pores. The size of the crystallites and of the pores varies depending on the doping of the silicon and the manufacturing conditions between a few nanometers and a few micrometers. If the wavelength of light is very much greater than the size of the structures in the PS, the PS appears to the light as a homogeneous material ("effective medium") and its properties therefore can be described by specifying an effective refractive index, which is dependent upon the refractive indices of the silicon crystallites and of the material in the pores.
The structuring of PS by CMOS-compatible process steps has already been demonstrated. Interference filters made from PS, especially Bragg-reflectors and Fabry-Perot filters have already been manufactured as well and are known from M. G. Berger, M. Thonissen, R. Arens-Fischer, H. Munder, H. Luth, M. Arntzen and W. Theiss, Thin Solid Films 255 (1995), pp 313-316. It has been possible already to integrate Bragg-reflectors in a silicon photodiode as a color-selective layer. Furthermore, the conduction of light waves in wave conductors made from PS has been demonstrated.
Now, another possibility to vary the refractive index of the PS consists in filling the pores of the PS with another material instead of air in order to detect substances or to determine their concentration in solutions. This property of the PS has not been put to use heretofore in the state of the art. By using diaphragms with selective permeability on the surface of the PS it is possible to achieve selectivity versus selected substances.
Therefore, the problem of the present invention is to create an analytic process and an analytic device by which a substance can be detected or its concentration can be determined with the use of porous silicon.